Given that it's water bourne, does it end up in all the worlds water supply via cross aquafer transfer?

There is some uranium in pretty much all water; there is some everything in pretty much all water but not very much. Uranium is soluble under oxidizing conditions but precipitates out of solution in reducing conditions. So uranium leaching out of these spent bullets will migrate to the local water table but then get bound up in the reducing parts of the aquifer deeper down. The depth to reducing conditions can vary from 0 in swamps to several hundred feet. Generally, I'd expect oxidizing conditions to prevail for 50 to 100 feet at the top of a typical aquifer. To answer the question, the uranium isn't going to migrate very far.

Seems like a real concern, but what do we do to stop it? And any chance you know why our military uses these poisonous shells?

It's pretty easy to stop: don't use depleted uranium in munitions intended for land use. I don't see a particular problem in the Navy's use of the stuff for anti-missile defense at sea. At sea, the bullets are going to drop to the bottom, which is reducing except in very shallow waters. There is also a lot of dilution in an ocean.

For anti-missile and armor-piercing munitions, you like to use the densest material available to get maximum momentum. Lead has a density of 11.35, while uranium has a density of 18.95 so it's a lot better material for anti-armor weapons. Other materials could be used. Platinum has a density of 21.45 so it would be even better but even the U.S. military can't spend that kind of money on bullets. The real advantage of depleted uranium is that its military acquisition cost is zero. It's a byproduct of uranium enrichment to fuel or weapons grade.

Natural uranium contains less than 1% U-235 (it's either 0.3% or 0.5% if memory serves) while about 3% is needed for reactor fuel and 9% for bombs. After producing enriched uranium from natural uranium, you have a bunch of depleted uranium on your hands and no real use for the stuff.

The most likely replacement for enriched uranium in munitions is tungsten (wolfram for the world outside the U.S.) with a density of 19.3. But tungsten would cost money. Free stuff is better than stuff that costs money.

The article probably didn't mention any relief for the Navaho women huh? Another question: has cancer increased in the male population on the reservation?

This was a research note but I would guess that relief for the Navajos will be on the way. The easiest and quickest thing to do is to test all the wells in the area for uranium and stop using the ones that carry high concentrations. If there is no low uranium concentration well in a given area, a wellhead treatment system is certainly feasible. I can think of a couple of approaches that should work. A good model would be the well water arsenic abatement efforts on Bangla Desh, where money is a lot tighter than even on the Rez.

You would't expect cancer in boys, although other problems, like reproductive system development problems, are possible. A good analogy for this would be the case of DES daughters in the 50s and 60s.

Hope this helps.